This invention relates to a centrifugal separator for separating suspended substances (sludge) in waste water, prior to the measurement of the oil density in the waste water which is to be discharged outboard from a ship.
Usually, before discharging waste water outboard from a ship, a small sample of the waste water is taken in order to measure the oil density contained in the waste water. If the oil density of the waste water is at or below the legally specified level, the waste water discharge valve is opened and the waste water is discharged outboard. To measure the oil density in the sample waste water, use is made of an optical oil densitometer, which is based on the principle that the light transmission rate and light scattering rate vary with the oil density when light is passed through the sample waste water. However, if there are suspended substances such as rust in the waste water, the measured values of the oil density may be in error, and therefore the sample waste water should be cleared of such suspended substances prior to the measurement of the oil density using the optical oil densitometer.
There exists a certain time lag between the collection of a sample of the waste water and the opening/closing of the outboard discharge valve, for the waste water to flow to and reach the discharge valve from the sampling part; consequently due to this time lag, waste water having a high oil density may be discharged outboard. For this reason, the time span between the collection of the sample waste water and the measurement of the oil density should be made as short as possible; thus, for removing suspended substances in the sample waste water, the time required should be made even shorter.
For this purpose, conventionally a centrifugal separator, such as that shown in FIG. 3, has been used for separating and removing suspended substances, as a pretreatment prior to the measurement of the oil density.
The conventional separator includes a cylindrical bucket 2 mounted inside a cover casing 1; this bucket is rotated at high speed by means of a drive shaft 3. A waste water inlet pipe 4 extending straight down from the upper part of the cover casing 1 reaches almost to the bottom of the bucket 2. In the upper part of the bucket 2 is formed an annular-grooved separated waste water chamber 5, and the waste water inlet pipe 4 passes through the inside of the separated waste water chamber 5. A waste water outlet pipe 6 is connected to the chamber 5, and surrounds the inlet pipe 4. The suspended substances in the waste water supplied to bucket 2 by the waste water inlet pipe 4 are separated by being pressed against the inner surface of the wall of the bucket 2, by reason of the centrifugal force generated due to the rotation of the bucket 2. The waste water from which the suspended substances have been separated out, flows into the separated waste water chamber 5, where it is pressed to the inner surface by means of centrifugal force so as to be discharged outside through the waste water outlet pipe 6.
In the conventional centrifugal separator as described above, a gap 7 for preventing the waste water outlet pipe 6 from coming into contact with the rotating separated waste water chamber 5, is provided between the separated waste water chamber 5 and the waste water outlet pipe 6. Therefore, if cleaning water is supplied through the waste water inlet pipe 4 to clean the inside of the bucket 2, after the rotation of the bucket 2 has halted, this cleaning water flows out only through the gap 7 between the separated waste water chamber 5 and the waste water outlet pipe 6, and it never flows out of the waste water outlet pipe 6. (It flows out through the waste water outlet pipe 6 only when the bucket 2 is rotating.)
Accordingly, the suspended substances separated and deposited on the inner surface of the bucket 2 cannot be discharged or removed by pouring in cleaning water. The suspended substances separated and deposited on the inner surface of the bucket 2 can be discharged or removed only by disassembling the entire unit. This makes the job of discharging and removing suspended substances extremely complex and troublesome. Moreover, during the separation process, flammable gas in the waste water gushes out of the gap 7 between the separated waste water chamber 5 and the waste water outlet pipe 6, and it remains in the space between the cover casing 1 and the bucket 2, posing the danger of an explosion due to sparks, etc. generated by contact of the bucket with the cover casing 1 during operation. Therefore, when mounting this centrifugal separator for pre-treatment purposes on a ship, an inert gas such as nitrogen must be filled and enveloped in the cover casing 1 so as to make the mechanism explosion-proof. This requires a large financial investment.
Furthermore, the separated waste water from the waste water outlet pipe 6 in the conventional centrifugal separator is discharged under almost no pressure, thus requiring a transfer pump for moving this separated waste water to the oil densitometer. This, of course, is another problem.
Moreover, for separating suspended substances inside the bucket, the oil in the waste water adheres to the surface of several umbrella-like parts 8 which are mounted inside the bucket, thereby delaying the discharge from inside the bucket and resulting in prolongation of the time needed for separation and removal of suspended substances.
The present invention is designed to solve the foregoing problems encountered with the conventional centrifugal separator.